1. Field of the Invention
The invention relates to solid propellants and more particularly to inhibitor compositions for limiting the exposed burning surface of solid propellant grains.
2. Description of the Prior Art
Inhibitors are used in solid propellants grains to limit the exposed burning surface. Inhibitor compositions resist combustion and are applied as a thin layer to the surface where burning is to be initially prevented. By restricting the grain burning surface area, relatively constant propellant thrust or gas volume can be maintained during motor firing. This is particularly important in cartridge actuated devices and in gas generators which must supply a steady stream of gas for long periods.
Various polymeric materials and resin-base compositions have been used as inhibitors. Some previously used polymer inhibitors require a volatile solvent for application to the grain. Evaporation of the solvent during curing can produce voids in the inhibitor. Such voids at the inhibitor/grain interface during burning can produce erratic rocket motor performance or even explosions.
Double-base propellants are usually inhibited with a cellulose based plastic such as ethyl cellulose. Ethyl cellulose based plastics are applied conventionally in the form of several layers of an inhibitor wrap or tape using an appropriate solvent system such as ethyl lactate/butyl acetate (50/50) to bond the inhibitor to the propellant grain.
Ethyl cellulose type inhibitors work well enough with an internal burning propellant grain. However, in an end-burning grain, such inhibitors permit minute propellant side-burning at the propellant/inhibitor/flame juncture. This side-burning produces an increasing burning surface area described as coning during motor firing which causes increasing chamber pressures in the rocket motor, as well as higher mass flow and correspondingly higher thrust. Such changes of pressure, mass flow, and thrust can be intolerable for systems requiring precise rocket ballistic behavior for proper operation.